Recently, multi-hop relay technology is widely discussed in the 3rd Generation Partnership Project Long Term Evolution-Advanced (3GPP LTE-A) and Worldwide Interoperability for Microwave Access (WiMAX), which both choose OFDMA as the downlink transmission scheme. In a relay-assistant system, each user equipment (UE) may access a base station eNode-B(eNB) or a relay node (RN) based on certain UE access principles such as maximum receiving power, minimum distance. Therefore, three kinds of links are formed. The link between an eNB and its serving UE (also named a macro UE) is called as a direct link, the link between a relay node (RN) and its serving UE (also named a relay UE) is called as a relay access link, and the link between an eNB and a RN hosted by the eNB is called as a relay backhaul link. With respect to in-band relay, these three kinds of links occupy the whole frequent band of a cell. By using a RN on the cell edge and utilizing the cell's edge frequent band to serve the relay UE, it's expected that not only the relay UE will not suffer inter-cell interference but also the received signal power from the serving RN is improved.
In a relay enhanced multi-cell communication system, the inter-cell interference (ICI) is the main source of downlink interferences. Some inter-cell interference coordination (ICIC) technologies have been proposed based on fractional frequency reuse (FFR). In the FFR, the cell edge UE (i.e. the relay UE) is only allowed to operate on a small portion of all available sub frequency bands. The portion of the frequency bands is allocated such that neighboring cells' edges may operate on mutually orthogonal frequency bands in order to avoid inter-cell interference. UEs that are in the center of a cell (i.e. macro UE) will operate on all or major portions of the available frequency band without any orthogonal limitation, since such UEs are closer to the eNB and thus are free of co-channel interference. When rely is considered, a variation of the FFR is called as partial frequency reuse (PFR), which divides the whole frequency band into two portions. The portion of the frequency band with a reuse factor 3 is called as cell edge frequency band (for the relay UE) and the portion of the frequency band with a reuse factor 1 is called as cell center frequency band (for the relay backhaul and the macro UEs).
FIG. 1 illustrates a schematic drawing of a static ICIC scheme in prior arts.
Generally, the cell edge frequency band is allocated to relay UEs, the cell center frequency band is allocated to macro UEs, and the relay backhaul link may share the cell edge frequency band with the relay UE or share the cell center frequency band with the macro UE. For example, as shown in FIG. 1, three sectors, i.e. sector 0, sector 1 and sector 2, are included in the cell served by the eNB, where a cell edge frequency band F0 is allocated to relay UEs of the sector 0, a cell edge frequency band F1 is allocated to relay UEs of the sector 1, a cell edge frequency band F2 is allocated to relay UEs of the sector 2, and cell center frequency bands F3 are allocated to macro UEs of sectors 0, 1 and 2. The frequency bands F0, F1, F2 and F3 are orthogonal with each other.
FIG. 2 illustrates a typical LTE FDD downlink frame structure.
As shown in FIG. 2, for a typical LTE FDD system, each downlink frame has a length of 10 ms and consists of 20 time slots (numbered from 0 to 19) of 0.5 ms. A subframe is defined as two consecutive time slots where subframe i consists of time slots 2i and 2i+1. In case of 10 MHz system bandwidth, there are total 50 PRBs (physical resource block) in each subframe.
FIG. 3 illustrates a diagram of resource allocation of the static ICIC scheme in case of the frame structure shown in FIG. 2.
As shown in FIG. 3, it is assumed that the relay UEs of sectors 0, 1 and 2 are allocated with 6 PRBs respectively, i.e. the cell edge frequency bands F0 (corresponding to PRB1-PRB6), F1 (corresponding to PRB7-PRB12), F2 (corresponding to PRB13-PRB18) are allocated to the sectors 0, 1 and 2 respectively, and that the macro UEs of sectors 0, 1 and 2 are allocated with 32 PRBs, i.e. the cell center frequency band F3 (corresponding to PRB19-PRB50) is allocated.
However, considering that the UEs are not always uniformly distributed and there may be significant differences between the loads of each relay, the static ICIC scheme equally allocating bandwidth resource within the cell edge limits the use of available spectra within the edge region and reduces the spectra utilization efficiency. By way of the example shown in FIG. 3, when the number of relay (cell edge) UEs within sector 1 is much larger than that of relay (cell edge) UEs within sector 2, serious overload will appear in the sector 1, whereas there may even be unused vacant physical resource blocks (PRB) such as vacant PRB 6, PRB 13 and PRB 14 in the other two sectors. In this case, a dynamic ICIC scheme is required to improve the spectra utilization efficiency